1. Field of the Invention
This invention relates to electronic power converters. More particularly, this invention relates to control of the input and output of electronic converters. Still more particularly, this invention relates to a system and apparatus for controlling electronic power converters using, random pulse width modulation.
2. The Prior Art
Electronic power converters include AC/DC converters also known as rectifiers and DC/AC converters also known as inverters. Typically, an electronic power converter is a network of semiconductor power switches. The fundamental component of output voltage of an electronic power converter is controlled by opening and closing individual switches in the network. The opening and closing of the individual switches is controlled by pulse width modulation (PWM) of switching signals applied to the individual switches. The distribution of the switching signals in time is referred to as a switching pattern. PWM may also be used to make current drawn from an AC supply source sinusoidal and in phase, with the supply voltage which maximizes utilization of the current in a rectifier. In inverters, appropriate width modulation of the switching pulses reduces the distortion of the output voltage and current.
One example of a system that uses electronic power converters controlled by PWM is an adjustable speed AC drive. Typically, an adjustable speed AC driver includes a rectifier, a DC link, an inverter and an AC motor. The rectifier converts input AC voltage from a power grid into a DC voltage. PWM or phase control may be used to control the rectifier. Alternatively, the rectifier may be uncontrolled. The DC link connects the rectifier to the inverter. The DC link is a capacitive or inductive-capacitive low-pass filter. The inverter receives the DC voltage from the DC link and converts the DC voltage to a three-phase, adjustable frequency, and adjustable magnitude AC voltage. PWM is used to control the inverter. The AC voltage is then applied to the AC motor.
In most electronic power converters controlled by PWM, the switching frequency is constant. The switching frequency is the number of switching cycles per second and per switch. In a rectifier, the switching frequency is typically two orders of magnitude higher than the input frequency. In an inverter, the switching frequency is typically two orders of magnitude higher than the output frequency. The switching cycles are also typically coincident with sampling cycles of a digital pulse width modulator controlling the converter. The coincidence of the switching and sampling cycles makes the switching frequency constant substantially equal to the sampling frequency.
Coincident cycles are a solution of convenience for makers of electronic power converters. However, coincident cycles may cause many problems. One problem with a constant switching frequency is the constant switching frequency results in clusters of higher harmonics in power spectra of voltages and currents of a converter at multiples of the frequency. These harmonics may cause undesirable side effects in systems connected to the input and outputs of the converter. For example, an adjustable speed AC drive having an inverter that is pulse width modified may have harmonic torques and forces generated in the motor that produce tonal noise and increase susceptibility of the drive to vibration.
Harmonic components of current from a power grid may cause electromagnetic interference (EMI) that is radiated into space surrounding the converter and is conducted to the grid. The EMI is concentrated in distinct narrow bands and may disrupt operations in sensitive communications equipment exposed to the EMI. These disruptions may be reduced by adding a special filter that may increase the expense and complexity of the equipment. There is a need in the art for a method of controlling an electronic power converter with PWM that reduces this and other problems.
Random Pulse Width Modulation (RPWM) techniques have been used to control electronic power converters. RPWM techniques have been described in xe2x80x9cRandom Pulse Width Modulation Techniques for Converter-Fed Drive Systemsxe2x80x94A Reviewxe2x80x9d A. M Trzynadlowski et al., IEEE Transactions on Industry Applications, Vol. 30, No. 5, pp. 1166-1175, 1994. An RPWM technique is characterized by random variations of the switching frequency. The random variations of the frequency alleviate undesirable components in PWM electronic power converters. Specifically, the fundamental AC component in PWM higher harmonics remains unchanged. However, the spectral power, measured in Watts, is converted to continuous power density, measured in Watts per Hertz, instead of being concentrated in the higher harmonics. The power spectra of the output voltage and current from a RPWM power converter emulate the spectrum of white noise. Consequently, spurious phenomena are significantly mitigated. Some examples of spurious phenomena mitigated include tonal acoustic noise, radiated EMI, and conducted EMI.
In most applications, a PWM converter is part of a larger control system. An example of these control systems is a digital speed or position controller of an electric drive. In order to provide PWM, a modulator connected to the converter must be provided with a reference signal of the output voltage prior to the beginning of a switching cycle in which a voltage will be generated by the power converter. This signal is typically produced by a control system in a sampling cycle preceding the switching cycle for which the signal is used. Thus, period of coincidental sampling and switching cycle are of the same length whether the sampling cycle is constant or of random length. This allows a modulator to receive signals for the reference voltage in time to compute and generate a switching pattern for the next switching cycle. It is a problem in systems having a random length of switching periods that this method requires the sampling cycle to vary by the same amount as the switching cycles. The use of identically varying sampling and switching cycles is easy to implement. However, these cycles are inferior to systems with constant sampling frequencies. A constant sampling frequency is superior because the constant frequencies represent a fixed level representing an optimal trade-off between various operating requirements. For this reason, RPWM has been a less desirable alternative for providing PWM in electrical power converters. Therefore, there is a need in the art for a system that provides the advantages of RPWM while taking advantage of constant sampling cycles.
The above and other problems are solved and an advance in the art is made through a method and apparatus for random pulse width modulation of an electronic power converter in accordance with this invention. In accordance with this invention, a method for Random Pulse Width Modulation (RPWM) is provided having constant periods for sampling cycles and random periods for sampling cycles. This method of RPWM is flexible and allows shaping of the frequency spectrum of the output voltage and input current of a power converter. This shaping optimizes the mitigation of spurious side effects.
In accordance with this invention, a modulator having a processor or microprocessor executing instructions stored in memory provides a method of RPWM. Alternatively, the modulator has circuitry that performs these steps.
RPWM is provided in the following manner. The method begins by determining a switching period randomly for a switching cycle subsequent to a current sampling cycle. The method then determines a switching pattern from a reference voltage generated from sample signals detected and processed in the current sampling cycle. Switching signals are then generated to make a specific switching pattern. The switching signals are then transmitted to switches of the converter during the switching cycle.
In accordance to the method of this invention, the switching period may be determined by calculating the switching period. In one exemplary embodiment, the calculation of the period may start by generating a first random number. A first delay between the switching cycle and a coincident sampling cycle is determined by multiplying the random number by the period of the coincident sampling cycle. A second random number is generated and multiplied by the period of a sampling cycle to determine the second delay. The second delay is the delay for the switching cycle subsequent to the cycle being calculated. The period of the switching cycle is then calculated by subtracting the first delay from the period of the sampling cycle and adding the second delay to the result.
The start time of the switching cycle is then calculated by adding the first delay to the start time of the coincident sampling cycle. The switching signals are then transmitted to the converter from the start time until the calculated period expires.
Sometimes the period of the switching cycle may be too short for calculation of the switching signals. Therefore, a comparison of the calculated switching period with a specific minimum allowable value of this period may be required. This minimum allowable value is hereinafter referred to as a switching period minimum. If the calculated switching period is less than the switching period minimum, the switching period is changed to the minimum switching period. If the period is changed to the minimum switching period the delay for the subsequent switching period must be adjusted. In the exemplary embodiment, the calculation of the second delay for the subsequent switching period is completed by subtracting the sample period from the switching period and adding the first delay.
The reference voltage may be received from a control system that generates the reference voltage from sample signals received in a previous sampling cycle.